Energy storage systems, including electrochemical devices such as Lithium-ion batteries having high energy densities, increasingly have to comply with safety requirements to meet the growing demand for large-size electrochemical cells.
One of the most critically important components to ensure safety of an electrochemical cell is the separator, whose primary function is to prevent physical and electric contact between the positive electrode and the negative electrode of the electrochemical cell while permitting electrolyte ions to flow there through.
Basically, two types of separators can be used: either porous ones, wherein a solution of an electrolyte in a suitable solvent fills the porosity of the separator, or non-porous ones, which are generally either pure solid polymer electrolyte (i.e. electrolyte dissolved in a high molecular weight polyether host, like PEO and PPO, which acts as solid solvent) or gelled polymer electrolyte system, which incorporates into a polymer matrix a liquid plasticizer or solvent capable of forming a stable gel within the polymer host matrix and an electrolyte.
The separator must be chemically and electrochemically stable towards the electrolyte and the electrode materials and must be mechanically strong to withstand high tensions generated during battery assembly operations. Also, its structure and properties considerably affect battery performances, including energy density, power density, cycle life as well as safety.
For high energy and power densities, the separator is required to be very thin and highly porous while still remaining mechanically strong.
For battery safety, the separator should be able to shut the battery down when overheating occurs so that thermal runaway, causing dimensional shrinking or melting of the separator, which results in physical contact of the electrodes, and the resulting internal short circuit can be avoided.
Also, a low thickness of the separator is required for high energy and power densities. However, this adversely affects the mechanical strength of the separator and the safety of the battery thereby provided.
One particular challenge always desired has been to provide a film separator or membrane separator with enhanced ionic conductivity allowing an excellent performance at higher charge/discharge cycle rates. In particular, in the case of hybrids inorganic-organic systems where safety is assured by the inorganic part, a higher performance of the membranes is highly desired.
There is thus still the need in the art for an alternative process for manufacturing membranes having high ionic conductivity to be suitably used as separators in electrochemical devices while maintaining outstanding thermo-mechanical properties during operation of the same.